A gunman shot dead a rival of Pakistani Taleban leader Baitullah Mehsud on Tuesday, dealing a potential blow to a government plan to defeat the Al-Qaeda ally. The murder news came as the military prepares an offensive against Mehsud, who has been accused for a string of bomb attacks and the assassination of former Prime Minister Benazir Bhutto in 2007. The militant commander, known as Qari Zainuddin, had recently spoken out strongly against Mehsud. He was killed in the northwestern town of Dera Ismail Khan, police said. “I confirm that Qari Zainuddin has been shot dead, but is not clear who is behind the killing, said “ Salahuddin, superintendent of police in the town, told Reuters. Militants are split into several factions in northwest Pakistan, some of which are rivals. The military went on the offensive against Taleban fighters allied with Mehsud in the Swat valley, northwest of Islamabad, in early May and they are in the final phase of that operation, the army has said. The offensive in Swat came after Taleban gains raised fears for the future of nuclear-armed Pakistan, a vital ally for the United States as it strives to defeat Al-Qaeda and stabilize Afghanistan. The government has also ordered an offensive against Mehsud in his South Waziristan stronghold near the Afghan border. In recent days, the military has been launching air strikes on Mehsud's bases while soldiers have been securing the main road into the mountainous region populated by ethnic Pashtun tribes. The United States has offered a reward of $5 million on information leading to Mehsud's location or arrest.Suspicion for his murder is bound to focus on Mehsud, who was accused of being behind the killing of a prominent anti-Taleban cleric in a suicide bomb attack in the city of Lahore this month. A security analyst said Zainuddin's killing was a setback for government efforts against Mehsud, but authorities should not depend on Mehsud's rivals to get rid of him. “He is Al-Qaeda number one in Pakistan,” said Mahmood Shah, a former security chief in the country's Federally Administered Tribal Areas (FATA) in the northwest. “This chap is too strong and a strong strategy is needed to deal with him.” The fighting in Swat sparked an exodus of civilians and aid workers fear a flow of villagers out of South Waziristan when fighting intensifies there.